Synthesis of Gamma-Alumina Nanoparticles by Freeze Drying

Synthesis by freeze-drying basically consists in rapidly freezing an aqueous salt solution containing the desired cation and the further sublimation of ice under vacuum conditions. After a conventional thermal treatment, the oxide nanoparticles are obtained. The influence of the parameters involved in the synthesis of γ-Al2O3 nanoparticles by a freeze-drying method have been studied: type of precursor salt (chloride and sulphate), salt solution concentrations (0.76-1.40 M), freezing rate and thermal treatment. Amorphous aluminium sulphate spherical granules with diameters ranging from 1 to 100 μm have been obtained. This compound decomposes at 825ºC, leading to the formation of γ-Al2O3. These porous granules are constituted by soft agglomerates of nanoparticles with primary particle size lower than 20 nm and values of the specific surface area of 120-180 m2/g. By optimizing the dispersing conditions a minimum particle size < 30 nm is measured, thus confirming that granules can be easily dispersed into nanoparticles. The thermal treatment determines the crystalline degree of the γ-Al2O3 and the surface area. A multimodal porosity is always obtained. The mesoporosity with mean size of 10 nm, attributed to the interparticulate porosity is practically constant, and the interagglomerate porosity (100-600 nm) strongly depends on the salt concentration, freezing rate and thermal treatment of the powders.